Final answer:
In haircutting, the guideline refers to a previously cut section which serves as a reference for the rest of the haircut to achieve a uniform length and style.
Step-by-step explanation:
When proceeding through subsections of a haircut, the guideline is always a previously cut section. This is a constant reference that hairdressers and stylists use to ensure the haircut is even and symmetrical. It is essentially a template for length that you've already cut which guides you as you continue cutting the rest of the hair.